|Lost Odyssey Enemy Boss|
|Wind, Aerial Killer|
|Quivering Sound Waves||Reverses all positive effects on all party members.|
|Cube||Deals 98-99% of current HP on all party members|
|Sneeze||Completely destroys party GC|
|Shadowus||Not too much damage but can cast multiple times in one turn|
|All-Curse||Curses all party members|
|Generatus||Very low chance of casting that spell, if you cast Reflect chance will increase, constantly heals part of Immortals HP.|
|Physical Attack||Hits very hard|
The Immortal One is the final boss in the Backyard side quest. Defeating the Immortal One will have the Backyard manager reward you with a Slot Seed and if you meet the secret condition, finish in under 30 turns, you will be given the coveted Quad-Element Seal. Finishing with 3 stars you can claim the Backyard White Book, Siderite Sword and Siderite Ring.
Well, this is the hardest boss in whole game. Don't even try to kill him until you are at the very least level 75+ with all Immortals. There are a lot of strategies for this one. First of all get ready for this fight. Make sure all your characters learned HP Max Up 1, 2, 3, 4 and same with MP. If you have the money, go to Kelolon Village and buy as many Heal Full and Goddess Medicine as possible. Make sure that at least Sarah and Ming got Tolten's Complete Defense skill, or Crisis Defense. You'll need the Reversa and Divide spells, and having Ailment Resistance on your Immortals won't hurt either. Take Sed in your party, as he is the most useful in this fight of all mortals.
First turn: Double Cast Divide with Sarah and Ming, Double Item Power Drink with Sed and prepare to use the spell Faster on Seth and Kaim. DO NOT USE BLACK MAGIC! It will deal 0 damage on him, so don't waste the valuable time. The first thing the Immortal One will do is most likely to Sneeze on you, destroying your GC bar. Don't bother with restoring GC, as he will just Sneeze on you again. The Immortal One has a nasty spell called Cube, which decreases your party's hp to 1-2%. To counter, use Complete Defense and Double Cast Reversa. When you get the Immortal One's HP low enough, he will start casting Shadowus 3-5 times on one turn. Unless you have Complete Defense activated, resurrecting may be dangerous. If your Immortal(s) dies, they will resurrect in two turns. If Sed (or your choice of mortal) dies, only resurrect them if they won't die instantly when the Immortal One attacks next time. The Immortal one can also cast Divide on your party, killing or nearly killing your party.
Although Black Magic spells are useless when cast directly at the boss you can cast Aqua/Flare/Ground/Wind Mine spells on your party, which will do about 3k dmg to him everytime he hits someone, depending on your party's level.
Additional experiences from another player: when you heal fully with Kaim or Seth (9999 health) he will be more inclined to throw a cube at you. All-generatus is very nice to have and keep your party above 4000, better yet 6000 in case he pulls out 3 shadowuses on you. Be sure to equip double-item before you start the battle, for me it was not worthwhile to revive Sed just for that. Next try, I will put Cooke in the team so she can help with Dividing and casting support when needed. Like the good tip mentioned above: when he has hit you with Cube, put Kaim and Seth in Complete Defense mode, so you can treat him on some nice Reversa in the turn thereafter.
When the Immortal One falls, you can feel proud. You just beat the hardest boss of Lost Odyssey.